Oil sand deposits such as those found in the Athabasca Region of Alberta, Canada, generally comprise water-wet sand grains held together by a matrix of viscous heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules which contain a significant amount of sulfur, nitrogen and oxygen. Oil sands processing involves mining the oil sand, bitumen water extraction and bitumen froth treatment to produce diluted bitumen which is further processed to produce synthetic crude oil and other valuable commodities.
Extraction is typically conducted by mixing the oil sand with steam, hot water and caustic. After extraction, the froth is initially stored in a large capacity froth storage tank until a sufficient volume is collected for subsequent froth treatment. The tank also acts as a surge vessel to absorb sudden fluctuations/changes in production rates. A froth storage tank is typically flat-bottomed (FIG. 1). However, the residence time within the tank is of sufficient duration to settle a portion of the solids from the froth. Build-up of solids within the tank may reach up to 30% by volume, such that the tank must be operated in the level of about 30-88% of capacity. Removal of the settled solids from the tank using a pump is often unsuccessful due to the loss of the solids slurry/froth interface. This loss is attributed to the uneven settling of solids on the bottom of the tank coupled with uneven removal resulting from coning of the solids. The solids build-up reduces both the capacity and ability of the tank to act as a surge vessel. The uneven deposition of solids can result in periodic sloughing of solids into the froth treatment process during times of low tank levels or large rate changes, causing major upsets in downstream equipment including, for example, overload of centrifuges and filters. Cleaning of the tank is typically conducted by cutting a hole through the tank sidewall and removing the accumulated solids using a loader to alleviate the problem temporarily for about two to three months. In addition, conventional froth storage tanks tend to have limited mixing capability.
Accordingly, there is a need in the art for an improved process and apparatus for storing and separating bitumen froth.